Is there any audio processing going on besides decoding for instance dts hd to pcm 7.1. Maybe becouse of Siri. There are lots of posts arround the web with people saying audio on Apple tv 4/4k is not good and recomend other devices if audio quality is top priority. I have a DD 5.1 file that is much lower volume on the Apple tv in mrmc vs my pc that is also decoding and sending pcm. But i also have lots of dts hd files that sound Identical.

Does mrmc do the decoding og is it tvos or both? Maybe mrmc decodes and sends it along and one can hope tvos just lets it pass trough or how does it work?

We don't do any intentional audio processing. If passthrough is enabled, DD 5.1 goes right to the audio receiver/amp for decode. The receiver/amp should indicate it is getting a DD 5.1 signal. Same for DTS 5.1.

Where things get complicated is for HD audio of 5.1 or 7.1 channels. I say HD audio as AC3 (DD 5.1) != EAC3 (DD+ 5.1). tvOS does not have an API to passthrough the encoded audio signals so we have to decode them internally to PCM, then send that out. Now if your PC is sending HD audio via passthrough, then your receiver/amp might be doing extra things to the HD audio after decoding.